Gravure printing and coating methods are well known in the art for applying liquids to webs or sheets. For example, U.S. Pat. No. 4,373,443 discloses a gravure cylinder for providing ink in a newspaper press. Cells or depressions engraved upon the surface of the gravure cylinder are filled to excess with a coating liquid that is used to form an image on the newsprint. Typically, a gravure cylinder rotates in a pan holding a constant level of coating liquid for wetting as taught in U.S. Pat. No. 3,936,549. A doctor blade, made of a metal softer than that of the surface of the gravure cylinder, wipes any excess liquid from the surface of the gravure cylinder so that only the engraved areas hold liquid. The gravure cylinder then delivers a precise amount of liquid to a web or other receiving surface upon contact with the engraved areas. The transfer typically occurs in a nip formed between the gravure cylinder and an impression roller that has an elastomeric cover serving as a backing for the web. The impression roller presses the web against the gravure cylinder to create a small area of contact. Alternatively, the web can be drawn against the gravure cylinder by web tension to create a nip.
U.S. Pat. No. 5,426,588 discloses a method for etching a gravure cylinder with a desired engraved pattern. Japanese Patent Document JP03114564 discloses a continuously etched gravure cylinder without a non-engraved surface area which reduces the doctoring concerns. Patch lengths can be controlled by controlling the cylinder cycling duration and frequency so that it is applying discrete patches at a duration and frequency specific to a given product. It should be possible, therefore, to provide variable patch lengths by cycling a continuously etched gravure cylinder with a desired engraved pattern into and away from the nip between the gravure cylinder containing the coating solution and the substrate to be coated. Cycling can be achieved by various means such as a pickup device or encoder driven signal off the central drive or other suitable drive member, (U.S. Pat. Nos. 3,762,319 and 6,272,986 B1), by following a registration mark printed in the first cylinder or existing on a cylinder (U.S. Pat. No. 1,096,483), or even based purely on time between actuation of sequential cylinders (U.S. Pat. No. 4,305,332). The cyclical spacing of the gravure cylinder engagement thereby facilitates variable patch lengths, not restrained to only those accommodated by a given machine frame. The cyclical timing of the gravure cylinder eliminates the necessity of wiping the gravure cylinder to the meticulous extent required for discontinuous patches without residual contamination in uncoated areas and under or over subsequently coated patches enabling the use of a continuously etched gravure cylinder with a relatively low doctor blade pressure. The cyclical timing driven transition into and out of the coating application required for a patch may take several inches to a foot thereby producing coated waste. The coated waste results in an impractical spool length, which increases manufacturing cost.
Standard practice is to use a precisely engraved patch on the gravure cylinder and subsequently register and progress cylinders to achieve the discrete patches with minimal extra transition lengths. It will be appreciated that it would be highly desirable to have a method for cycling the gravure cylinder that provides transitions equal to the 2 mm transitions currently achieved with standard gravure patch application.
An attempt at such an approach is described in Japanese Patent Document JP 03114564 wherein, while the top side of a continuously running substrate is in a free state, the bottom side is coated by a gravure roll having a engraved area covering entire periphery. Being freed from excess coating material from the surface by a doctor blade, the gravure roll rotates with a peripheral speed in relation to the substrate speed, and is fed with a certain amount of coating material in the engraved part. The substrate is dried. Then, while the top side of the continuously running dried substrate is in a free state, the bottom side is coated by a continuously engraved gravure roll. Being freed from excess coating material from the surface by a doctor blade, the gravure roll rotates with a peripheral speed in relation to the substrate speed, and is fed with a certain amount of a different coating material in the engraved part. The substrate is then dried.
The multicolor gravure coating apparatus has a plurality of combination units of a gravure roll having a diameter of about 20-50 mm, an engraved part covering the entire periphery, and applies a coating material to the bottom part of a continuously running substrate while the top part is in a free state. A doctor blade scrapes off excess coating material from the gravure roll surface to feed a certain amount of coating material to the coating part. A drying part dries the coated material. Each doctor blade-gravure roll combination is installed independently and can be freely moved to and from the bottom of the substrate.
While a continuous substrate is running, by using a number of combinations comprising a gravure roll, doctor blade and drying unit, a number of coating materials of different color can be applied. Namely, in each unit, while the top part of a continuously running substrate is in a free state, a certain amount of coating material is applied to the bottom part of the substrate by a gravure roll of about 20-50 mm in diameter. Metering of this coating material is done by a doctor blade that scrapes off excess coating material from the gravure roll surface. The solid coating formed on the bottom part of the substrate is dried while passed through the drying part. By repeating the coating and drying, multicolor coating is obtained.
Japanese Patent Document JP 03114564 provides an example of a multicolor gravure coating apparatus wherein three sets of coating unit and drying unit combinations are installed in the running direction of continuous substrate for forming coatings of three colors. In the coating unit, the substrate is stretched by two tension rolls and runs horizontally. A gravure roll having engraved part covering entire periphery is located transversely below the middle part of the substrate between the tension rolls. The gravure roll is supported and rotated freely by the bearings supported by a pair of supports that can be moved up and down by an operating mechanism and are installed on a base. Rotating force is transmitted via a coupling from an operating motor. In this example, at the contact point, the gravure roll rotates in a direction opposite to that of the substrate. The outer peripheral surface of the gravure roll has an engraved part that is narrower than the width of the substrate. An overflow receiver is fastened by a bolt to the base below the gravure roll. A coating material feed nozzle that feeds the coating material to the gravure roll is fixed on the overflow receiver. Each coating part unit is installed in such a way that it can be freely moved independently to and from the bottom surface of the substrate by a lifting mechanism.
For achieving good coating by each coating unit to the substrate, speed control rolls are installed upstream and downstream of the coating unit against the running direction of the substrate, and a tension detection sensor is installed further upstream (or downstream, as the case may be) of the speed control roll. This tension detection sensor detects the tension of substrate fed to the coating unit, and each speed control roll controls the running speed of the substrate passing the gravure roll portion of the coating unit according to the tension detected by the tension detection sensor. Each speed control roll is under digital control and rotates at the same speed.
A problem with such a procedure is the inability to achieve the transitions within the tolerances that are achieved with a standard gravure patch application procedure predominantly used to make dye donor ribbon. Smearing can occur whenever there is relative lineal motion between the web and roller at the start or finish of a patch, and a coating line can occur at the finish when the web and gravure roller are separated. These conditions reduce product quality sometimes rendering the product unusable. Accordingly, it will be appreciated that it would be highly desirable to have a method for cycling the gravure cylinder that provides patch coating within the desired transition tolerances. It is also desirable to define the prerequisites for the cycling rate required to provide the transitions with the same accuracy as is delivered by standard gravure patch application methods, and to deliver the requirements to prevent any transients in the initiation or termination of the patch application on to the web. Although the travel speed of the web and gravure roll can be controlled with precision, a problem with smearing and uneven coating persists; so, there is still a need for a method for precisely coating patches on a web without smearing at the transitions.